Refer Chirpstack MQTT forwarder in LNS configuration for basic idea to configure MQTT forwarder, When you open Chirpstack >> MQTT forwarder add internal LNS server:port in Server and Save.

Read chirpstack documentation for MQTT forwarder, where MQTT server="tcp://" for connecting gateway to internal chirpstack LNS.

After adding server and saved it, gateway backend system will reset concentratord modulue to apply new configuration, System Log feature can have this information as shown below.

Once it has been reset/restarted succesfully, it will start functioning normally.

Gateway configuration is all set to move ahead into LNS configuration.

As like NodeRED, Chirpstack LNS is pre-installed in system as well as can be opened from same Applicaiton menu by clicking the icon or "<gateway_IP>:8080"

Chirpstack will ask credential to login into chirpstack console.

Credentials : user/pass --> admin/admin , Make sure to change it later for security purpose.

After login you will be redirected to Dashboard, where you can see gateway and end devices online/offline status, device dat-rate usage as well as Gateway Map.

in above data there in No Data, as no Gateways/Devices has been added.